The present invention relates generally to a reproducing apparatus and a signal processing apparatus each having a filter for eliminating cross modulation distortion if a cross modulation noise is caused by poor compatibility between an amplifier and a speaker connected to a reproducing apparatus for reproducing a digital signal of which quantization bit having high-frequency band is one bit.
The PCM (Pulse Code Modulation) signal for use in the compact disc (CD) is a digital signal which is limited in band to 22 KHz. As shown in FIG. 1, an RF signal reproduced by an optical pickup 51 from an CD 50 is converted by a signal processor 52 into a PCM signal. The PCM signal is then filtered by a digital filter 53 to steeply eliminate sampling noise higher than 22 KHz. The resultant signal is then converted by a D/A converter 54 into an analog signal to be supplied to an analog low-pass filter 55.
Recently, a digital audio signal has become a focus of attention which consists of one bit data obtained by performing delta sigma (xcex94xcexa3) modulation on an analog audio signal. This digital audio signal is hereafter referred to as a direct stream digital (DSD) signal. Compared with a conventional PCM audio signal having format of sampling frequency 44.1 KHz and data word length 16 bits for example, the DSD signal comprises a very high sampling frequency such as 64 times 44.1 KHz and a short data word length such as one bit for example, featuring a wide transmittable frequency band. In addition, in the audio band, which is low enough by performing sampling on such a high over-sampling frequency as 64 times 44.1 KHz, the DSD signal can ensure a high dynamic range. These features allow this signal to be applied to recording and data transmission for example of high tonal quality.
The DSD signal is also a one-bit pulse train having a signal component ranging to 1.4 MHz. Simply passing this signal through a low-pass filter provides an analog audio signal. As shown in FIG. 2 for example, an RF signal is reproduced through an optical pickup 61 from an optical disc 60 on which a DSD signal is recorded and the RF signal is converted by a signal processor 62 into the DSD signal. This DSD signal is then passed through a low-pass filter 63 to obtain an analog audio signal at an output terminal 64.
Referring to FIG. 3, there is shown a constitution of a xcex94xcexa3 modulator for obtaining the above-mentioned one-bit quantization data. This xcex94xcexa3 modulator comprises an adder 71, an integrator 72, a quantizer 73, and a delay circuit 75. The output of the adder 71 is supplied to the integrator 72. The output of the integrator 72 is supplied to the quantizer 73. The output of the quantizer 73 is taken out at an output terminal 74 and, at the same time, converted by the delay circuit 75 into a negative code to be fed back to the adder 71, in which the negative coded is added to the analog audio signal supplied from an input terminal 70. The output of the adder 71 is then integrated by the integrator 72. The output of the integrator 72 is quantized by the quantizer 73 in every sampling period. Consequently, one-bit quantization data can be obtained at the output terminal 74.
Meanwhile, in the case of the above-mentioned PCM signal, there is no fear that the components of higher than 22 KHz leak into devices in the succeeding stage because these components are steeply eliminated by the digital filter 53. In the case of a DSD signal, however, if the blocking by the analog low-pass filter 63 is insufficient, unnecessary high-frequency components may leak in the succeeding stage such as an amplifier or a speaker to cause cross modulation distortion, thereby deteriorating tone quality. If the above-mentioned undesired high-frequency components leak in a high amplitude into a speaker capable of reproducing with fidelity high-frequency tones of up to 100 KHz for example, the speaker may be damaged.
The undesired high-frequency components include a noise component that is shifted to the high-frequency side by a noise shaper when a DSD signal is recorded onto a recording medium such as an optical disc, a hard disc, or a magnetic tape. Currently, the degree of the noise shapers for generating a DSD signal is mainly 3 to 7. The low-pass filter must eliminate the high-frequency components, namely these noise components. However, in the case of a DSD signal, the characteristics of the noise shaper can be set at will in the recording system, so that it is unclear how much noise exists in which band. For example, there is an apparent difference in the quantity of undesired high-frequency component between the noise characteristic obtained by a 3rd-order noise shaper shown in FIG. 4 and the noise characteristic obtained by a 7th-order noise shaper shown in FIG. 5. The low-pass filter on the reproducing apparatus side cannot recognize this difference. Namely, when recording a DSD signal on an optical disc, a hard disc, or a magnetic disc for example, the reproducing apparatus side cannot determine whether the recording is through a 3rd-order noise shaper or a 7th-order noise shaper. Further, when a higher-order noise shaper is developed in the future, the currently available low-pass filters are not enough to block undesired high-frequency components.
Also, because DSD signal range as wide as 1.4 MHz, simply lowering the cutoff signal fc of the low-pass filter cannot fully make the most of the DSD signal performance. Consequently, it is hard to reproduce a DSD signal in the same manner as PCM signal reproduction by means of one low-pass filter.
It is therefore an object of the present invention to provide a digital signal reproducing apparatus that allows the selection of an appropriate low-pass filter according to the configuration of a succeedingly connected device and the high-frequency component of a digital signal consisting of one-bit quantization data to be reproduced and, therefore can prevent the undesired high-frequency component from leaking due to insufficient blocking into the succeeding stage, thereby preventing the cross modulation distortion from occurring in the succeeding stage.
It is another object of the present invention to provide a digital signal processing apparatus that allows selection of an appropriate low-pass filter according to the high-frequency component of a one-bit digital signal and can supply a signal with the occurrence of cross modulation distortion prevented to an amplifier.
In carrying out the invention and according to one aspect thereof, there is provided a reproducing apparatus for reproducing a recording medium recorded with a digital audio signal composed of a quantization one-bit data sampled at a sampling frequency fs (Hz) sufficiently higher than an audio band, comprising: a reproducing means for reproducing the digital audio signal from the recording medium; a signal processing means for performing predetermined signal processing on the digital audio signal reproduced by the reproducing means; a low-pass filter means having a plurality of different frequency characteristics with cutoff frequency above the audio band; and a selecting means for selecting one of the plurality of different frequency characteristics of the low-pass filter means according to a user""s operation.
In carrying out the invention and according to another aspect thereof, there is provided a reproducing apparatus for reproducing a recording medium recorded with a digital audio signal composed of quantization one-bit data sampled at a sampling frequency sufficiently higher than an audio band, comprising: a reproducing means for reproducing the digital audio signal from the recording medium; a signal processing means for performing predetermined signal processing on the digital audio signal reproduced by the reproducing means; a low-pass filter means having a plurality of different frequency characteristics with cutoff frequency above the audio band; a high-frequency component testing means for measuring a high-frequency component included in the digital audio signal processed in a predetermined manner by the signal processing means; and a selecting means for automatically selecting, on the basis of a measuring result of the high-frequency component testing means, one of the plurality of different frequency characteristics of the low-pass filter means.
In carrying out the invention and according to still another aspect thereof, there is provided a signal processing apparatus for converting into an analog signal a digital audio signal composed of a quantization one-bit data sampled at a sampling frequency fs (Hz) sufficiently higher than an audio band, comprising: a receiving means for receiving the digital audio signal composed of a quantization one-bit data sampled at a sampling frequency fs (Hz) sufficiently higher than an audio band; a low-pass filter means of which cutoff frequency for filtering the digital audio signal composed of a quantization one-bit data received by the receiving means is above the audio band and having a plurality of different frequency characteristics; and a selecting means for selecting one of the plurality of different frequency characteristics of the low-pass filtering means according to a user""s operation.
In carrying out the invention and according to yet another aspect thereof, there is provided a signal processing apparatus for converting into an analog signal a digital audio signal composed of a quantization one-bit data sampled at a sampling frequency fs (Hz) sufficiently higher than an audio band, comprising: a receiving means for receiving the digital audio signal composed of a quantization one-bit data sampled at a sampling frequency fs (Hz) sufficiently higher than an audio band; a low-pass filter means of which cutoff frequency for filtering the digital audio signal composed of a quantization one-bit data received by the receiving means is above the audio band and having a plurality of different frequency characteristics; a high-frequency component testing means for measuring a high-frequency component included in the digital audio signal composed of a quantization one-bit data received by the receiving means; and a selecting means for automatically selecting one of the plurality of different frequency characteristics of the low-pass filtering means on the basis of the measuring result of the high-frequency component testing means.